1. Field of the Invention
The present invention relates to a novel process for preparing compound oxide type superconductors, more particularly, a novel process for preparing superconductors of bismuth-containing compound oxides such as Bi--Sr--Ca--Cu system.
2. Description of the Related Art
The superconductivity is a phenomenon which is explained to be a kind of phase change of electrons under which the electric resistance becomes zero and the perfect diamagnetism is observed. Under the superconductivity, permanent current of high density flows without any loss. Therefore, when the technology of superconductivity can be applied to electric power transmission, power loss of about 7%, which is lost in the electric power transmission today, can be saved greatly. Development of superconductors is also expected in the field of measurement and of medical treatment such as NMR, .pi. neutrons medical treatment or high-energy physical experiments. In the applications of electromagnets for generating strong magnetic fields, the technology of superconductivity is expected to accelerate development of the technology of fusion power generation, MHD power generation, magnetic levitation trains and magnetically propelling ships.
Several superconducting devices have been proposed and developed in electronics. A typical application of the superconductor is Josephson device in which quantum efficiency is observed macroscopically when an electric current is passed through a weak junction arranged between two superconductors. The tunnel junction type Josephson device is expected to be a high-speed and low-power consuming switching device owing to a smaller energy gap of the superconducting material. Development of the superconducting devices such as high-speed logic units or of no power-loss wiring materials is also demanded in the field of high-speed computers in which the power consumption per unit area is reaching to the upper limit of the cooling capacity with increment of the integration density in order to reduce energy consumption.
The critical temperature "Tc" of superconductivity, however, could not exceed 23.2K of Nb.sub.3 Ge which was the highest Tc for the past ten years. The possibility of an existence of new types of superconducting materials having much higher Tc was revealed by Bednorz and Muller, who discovered a new oxide type superconductor in 1986 (Z. Phys. B64, 1986 p189).
It had been known that certain ceramic materials of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba--Pb--Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba--Bi type compound oxides also show superconductivity. These superconductors, however, possess rather lower transition temperatures of about 10K and hense usage of liquidized helium (boiling point of 4.2K) as cyrogen is indispensable to realize superconductivity.
The new type compound oxide superconductor discovered by Bednorz and Muller is represented by [La,Sr].sub.2 CuO.sub.4 which is called the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type compound oxides show such higher Tc as 30K which are extremely higher than known superconducting materials. It was also reported that C. W. Chu et al, discovered, in the United States of America, another superconducting material, so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of about 90K in Feb. 1987 (Physical Review letters, Vol. 58, No. 9, p 908).
Maeda et al reported the other type new superconducting material of Bi--Sr--Ca--Cu--O system (Japanese Journal of Applied Physics. Vol. 27, No. 2, p 1209 to 1210).
Bismuth type compound oxides are chemically much more stable than the abovementioned YBCO type compound oxide and have such a very important merit that high Tc superconductors of higher than 100K can be realized without using rare earth elements as a constituent element so that the production cost can be reduced.
The above-mentioned new types of superconducting materials can be obtained by solid reaction in a bulk form of sintered block which was obtained by sintering a powder mixture of oxides or carbonates of constituent metal elements which has predetermined atomic ratios.
They can also be deposited on a substrate in a form of a thin film by physical vapour deposition (PVD) technique such a RF sputtering vacuum deposition, ion-plating or MBE or chemical vapor deposition (CVD) technique such as thermal CVD, plasma CVD, photo-CVD or MOCVD.
In the case of production of bismuth type oxide superconductors, however, there is a special problem, because the vapour pressure of bismuth (Bi) is very different from the other constituent elements, so that it is very difficult to control the composition of the product. Still more, the products obtained by the conventional process are porous sintered articles having relatively lower density and being not homogeneous in quality. Therefore, it is requested to improve the quality of bismuth type compound oxides.
An object of the present invention is to overcome the problem of the conventional process and to provide a novel process for preparing high Tc bismuth type superconductors such as a Bi--Sr--Ca--Cu system of high quality easily.